Transformation of the critical state in hard superconductors resulting from thermomagnetic avalanches

Abstract

The results of experimental studies of magnetic flux dynamics in finite-size superconductors, obtained using integral and local measurements methods, are presented. Local methods were aimed at clarifying the role of the demagnetizing factor in the dynamic formation of a complex magnetic structure of the critical state of hard superconductors. To understand the reasons for drastic transformation of the magnetic induction, we further analyzed the literature data on the visualization of flux dynamics in the presence of avalanches, obtained by magneto-optical methods. New features in the behavior of the magnetic flux during and after an avalanche were revealed and characterized: two stages in the formation of the magnetic induction distribution inside the avalanche region were established—homogeneous and heterogeneous filling with magnetic flux; the mechanism of inversion of the induction profile; velocity oscillations in the propagating magnetic flux front; transformation of the critical state band near the edge of the sample; and the role of the thermal effects and demagnetizing factor in the dissipative flux dynamics. The generalized information allowed us to present, within the framework of the Bean concept, a model of the transformation of the patterns of magnetic induction in the critical state and superconducting currents in a finite superconductor occurring as a result of flux avalanches in two different regimes—shielding and trapping of magnetic flux.

Magnetic moment inversion at giant flux jump: dynamical property of critical state in type-II superconductors

“…In such case, the penetration of a flux avalanche results in the appearance of paramagnetic circular current in the diamagnetic region of a superconductor, that determines the local inversion of the magnetic induction profile (Fig. 26 in 33 ). On the other hand, in the case of an avalanche exit, i.e.…”

“…in the flux trapping regime, a circular diamagnetic current appears in the paramagnetic region of the superconductor (Fig. 27 in 33 ).…”

“…The construction of a scheme for inverting the magnetic moment in the case of an avalanche, presented on Fig. 3(b), was facilitated by the data of another researchers 27,28,33,35 , where the magneto-optical technique was used to study the transformation of the magnetic field induction pattern on the surface of a superconducting disk during the avalanche dynamics. Analysis of these data allowed us to draw the following conclusions: (a) the avalanche of the exit of trapped flux originates near the edge of the sample and its front propagates inward in the form of an arc, oriented by the convexity toward the center 27 ; (b) the value of the critical current at the front of the avalanche, in dynamic conditions, is close to its quasi-static value, and the direction is opposite to that of current near sample edges, which ensures the capture of the magnetic flux 27,33 ; (c) the avalanche front, encountering an obstacle, is deformed in accordance with its shape 35 .…”

“…3(b). On the boundary of the avalanche, a critical state layer with a current of diamagnetic direction opposite to the current providing magnetic flux capture emerged 33 . This current realizes the inversion of the magnetic induction profile in the central part of the avalanche spot (creates a “crater”, Fig.…”

Multi-Steps Magnetic Flux Entrance/Exit at Thermomagnetic Avalanches in the Plates of Hard Superconductors

“…Here, “irregular” jumps are the jumps leading to a complex, irregular distribution of the magnetic flux in the superconductor. The analysis of the visualization patterns of the flux dynamics, presented in the above papers, allowed, within the framework of Bean’s concept, to understand the mechanism of local inversion of the surface self-field in the volume of conventional NbTi superconductor [ 15 , 16 ] and the complete inversion of the magnetic moment in the YBa 2 Cu 3 O 7−δ single crystal [ 17 ].…”